Simple inheritance question

>I have a simple question regarding inheritance in a web form.
>
I have a DropDownList in an aspx form. It is called DropDownList1
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I have a class that will be overriding the render event so I have a
snippet of this class:
Public Class CustomDDL
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Inherits DropDownList
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Protected Overrides Sub render(ByVal writer As HtmlTextWriter)
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For Each li As ListItem In Items
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li.Text = li.Text & " - Test Render"
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Next
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End Sub
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End Class
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My question is ... what is the syntax to "convert" dropdownlist1 from a
dropdownlist to my CustomDDL?
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>
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Thanks!
>
Ron
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My question is ... what is the syntax to "convert" dropdownlist1 from a
dropdownlist to my CustomDDL?

RSH wrote:
>>
Short answer: You can't.
>
Longer Answer: You can't, and here's why.
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The DropDownList class has a bunch of properties and methods.
Your CustomDDL, because it inherits from this, has /all/ of these, plus
any new stuff that you've added.
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However, /because/ your CustomDDL has all the base class stuff, you can
"treat it as" a DropDownList, i.e. you can "up-cast" your control to "get
at" the base class functionality, as in
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DirectCast( customDDL1, DropDownList ).whatever( ... )
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The converse is /not/ True.
The DropDownList does /not/ have all the "stuff" that your customDDL has,
so you cannot "treat is as" one (i.e. you can't "down-cast" from the base
class to a subclass).
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However ...
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Object Variables are clever things these days. Not only can they hold the
/actual/ type that they are declared as, but they can hold an object of
any sub-class as well (because the subclass has /all/ the base class
stuff, plus bits, so can be "treated as" the base Type).
So, if in your base web page, you have
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Protected WithEvents DropDownList1 as DropDownList
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then there's nothing to stop you doing
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MyBase.DropDownList1 = New CustomDDL
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The base class is happy, because it has a reference to an object that it
can "treat as" a DropDownList, but when it tries to call, say, the
Render() method, it will be the /override/ of Render in your custom
subclass that actually gets called!
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Welcome the Wonderful World of Polymorphism!
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HTH,
Phill W.